Britain has agreed to head an international security force in Afghanistan and deploy more than 1,000 troops in the capital, Kabul, military sources told the Guardian last night.

A reconnaissance team led by Major General John McColl, commander of the army's third division, will conduct a detailed survey of Kabul at the weekend to assess the security situation there, the sources said.

The force is expected to total 3,000 troops, about half of them British. France and Germany have also said they are prepared to contribute several hundred troops to the force.

The US and the UN have been pressing Britain to lead the force ever since the agreement last week on the composition of an interim Afghan government, which is due to take office on December 22.

Tony Blair said after a meeting in London on Tuesday with Colin Powell, the US secretary of state, that he was willing in principle for Britain to head the force. However, an agreement was held up by concern among military commanders about the force's precise mission.

The government also told Washington that it would not be prepared to lead the UN mandated force unless the US agreed to provide back-up support. The force is expected to fly into Bagram airbase, near Kabul, early next week, once Gen McColl has reported back on the situation in the Afghan capital.

Admiral Sir Michael Boyce, chief of the defence staff, warned earlier this week that the Afghans did not want "anything that looks like an occupation army", but rather a military presence sufficient to reassure the interim administration that the international community is prepared to help stabilise the country.

The British contribution is expected to consist of a small headquarters team backed up by some 1,500 soldiers, including paratroopers. The force is expected to be replaced in spring by troops from Muslim states, including Turkey and Jordan.

Of the British troops already in Afghanistan, soldiers from the royal marines special boat service (SBS) were thought to be patrolling Kandahar yesterday. The troops, not in uniform but driving a Land Rover equipped with a heavy-calibre machine gun, were seen in the bombed-out streets of the city waving to bemused residents.

SBS personnel were initially deployed around Bagram airbase, but were moved south when royal marines relieved them a fortnight ago.